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4 Answers

Requires vertical separation for IFR traffic…

Asked by: 2638 views Airspace, FAA Regulations, Flight Instructor, Instrument Rating

Guys I need help figuring this out, I took my CFII a week ago and busted. One of things he got me on was required vertical separation for IFR traffic. I was able to locate the RVSM separation as well as the lateral separation for aircraft enroute. 

 

My understanding is 1,000' for opposite direction traffic in RVSM airspace. And 3nm lateral separation within 40nm of a radar antennae and 5nn beyond 40nm of a radar antennae. 

 

Nowhere in the AIM does it specifically say what the vertical separation standards are for IFR traffic not in RVSM airspace.

Thanks in advance to anyone who can help me out. 

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4 Answers



  1. KDS on Jun 09, 2019

    The answer is 1,000′ if it is below RVSM airspace and 2,000′ if it is above RVSM airspace. However, I’m stumped for a reference. But, I know there are a couple folks here who will be able to provide that.

    My guess is that you really did know the material at the level that would have satisfied him, but he asked the question in such a peculiar way that you were hunting for a deeper explanation.

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  2. Matt J Adams on Jun 09, 2019

    Thanks KDS, maybe you could also provide some insight into the discussion that led up to this topic in my checkride.

    We were talking about maintenance and the DPE asked me why it is important that our altimeter is within 75\’ of the field upon departure, he was trying to make the point that we are in mountainous area and will have 2,000′ of obstacle clearance. I explained to him once we begin an approach those 75\’ may make a huge difference as we descend. He just kept reiterating that he was talking about en route and not on an approach.

    The only logical thing I can think of after the fact is if your altimeter is say 100 feet high so you\’re actually 100 feet lower than you should be, but not only that maybe you\’re off your indicated altitude by 100 feet, this puts your true altitude 200 feet low. And assuming there were no standards, say the guy below you is in the same situation but he is actually 200 feet high, now you\’re in a near miss situation.

    After discussion with other CFIs at my school we are still unsure if the exact answer he was looking for.

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  3. John D Collins on Jun 10, 2019

    The check on the ground for difference from field elevation also relates to vertical separation. For altitudes below 18000 MSL, vertical separation from VFR traffic is only 500 feet and you are using up some of the budget for vertical separation. So if you remain at assigned altitude within +/- 100 feet and VFR traffic does the same, if both of your altimeters are off in the opposite direction by 75 feet, you have used up half of the 500 foot separation. That is before you take into consideration differences in altimeter settings. As far as terrain separation, the baro altimeter has significant potential uncompensated error due to temperature and the magnitude of the error increases with altitude above and distance from the location of baro altimeter setting. That is why the terrain clearance for IFR cruise increases to 2000 feet in mountainous areas, because 1000 feet is simply not enough. The baro altimeter is a poor terrain avoidance device and is why these tolerances are high. If you don’t want to hit the rocks, a GPS altitude is much better. If you want to maintain vertical separation from other aircraft, everyone using a baro altimeter either set to 29.92 in the flight levels or to a local corrected value below the flight levels is what is needed.

    Fortunately, the temperature effects are much lower when making an approach, because you are nearer the baro altimeter setting location, both in terms of distance and altitude above the location.

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  4. KDS on Jun 10, 2019

    John did a better job of answering the question than I could have.

    I know it’s not part of your question, but I’d just like to toss in an old story where two great pilots messed up. We’ve all heard that 1 inch is 1,000 feet of altitude bit. It was a military aircraft with two good pilots. There had been a big pressure change since the last time the aircraft flew. By bad luck, but pilots set the altimeter on their side to the wrong altitude. They didn’t discover the error until they were on approach at a Navy base. They broke out flying between the antennas on top of houses. To their credit, they were very open about their mistake and came back and talked extensively about it to everyone to include at the next safety meeting.

    I was always amazed that these guys who I held in such high esteem could make such an error. Then, many years later I was flying a recreational glider. Some glider people set the altimeter to field elevation and some set it to zero when they plan on coming back into the same airport. I had to reset the altimeter before takeoff. I took the tow to 3,000′ and released. As soon as I could quit concentrating on the tow plane and look at the ground I had a bad feeling. Either everyone had just made their houses bigger than they used to be or I had made the same mistake.

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